The present invention relates to a brake booster for automotive vehicles, and more particularly to a tandem brake booster the housing of which is subdivided into front and rear chambers by means of a partition wall assembled therein and in which front and rear movable walls are respectively assembled within the front and rear chambers and connected to each other through the partition wall, the front chamber being subdivided by the front movable wall into first constant and variable pressure chambers, and the rear chamber being subdivided by the rear movable wall into second constant and variable pressure chambers.
In such a conventional tandem brake booster as described above, a first passage for communication between the first and second constant pressure chambers is formed in the peripheral wall of the housing, and a second passage for communication between the first and second variable pressure chambers is also formed in the peripheral wall of the housing. In such an arrangement of the first and second communication passages, the construction of the brake booster becomes large in size, and the responsibility in operation of the brake booster is inevitably influenced by the length of the passages. To solve the problems, an improved tandem brake booster has been proposed in an Early Publication of the Japanese Utility Model Application No. 58-106265, as is illustrated in FIG. 1. In the improved tandem brake booster, a first passage 3 for communication between first and second variable pressure chambers 1 and 2 is formed by an axial passage 5a in a hub member 5 of the front movable wall 4 and an axial hole 7a in a hub member 7 of the rear movable wall 6, while a second passage 10 for communication between first and second constant pressure chambers 8 and 9 is formed by a radial hole 11a of an inner cylindrical case 11 coupled within the front hub member 5 and axial and radial holes 5b, 5c in the front hub member 5. In this arrangement, however, the inner cylindrical case 11 is coupled within the front and rear hub members 5 and 7 and fastened to them by means of a clip 16 fixed to its rear end and a nut 17 fixed to its front end. Such an assembly process of the inner cylindrical case 17 is complicated, resulting in decrease of the productivity of the brake booster.